limited slip rear diff binding

[93turbo_animal]

Hi guys my father in-laws got a 02 f350 srw 4x4 with the 3.73 gears and hes got a bad binding problem at first we thought it was the GN ball to trailer but he keeps that greased real well so we road around in the back of the truck no binding coming from GN but from under neath so he changed the fluid and added the modifier from ford and no change he's only got around 40k-50k miles on it however most of those are pulling a GN trailer that when loaded he grosses out at 28k I told him he probably needed to add more of the friction modifier but I don't know any one here ever have this problem or any Ideas as to what to do to fix it Thanks

 

[Double-S-Diesel]

may need another dose of modifier, had the same problem in my 00 after installing amsiol